I have the MMV and can say that's an incredible set! I bought the Joust set this month and it haven't arrived yet (usually takes 40 days to arrive here in Brazil ) but it looks fantastic! It's a tough choice.

As stated they are both cool, maybe MMV a bit more awesome , so I would go for that. And that we will still have Joust for some years I think but MMV is a Castle/fantasy era set from 2008 so it's time will soon end. Get it before they are sold out on Lego because then the price will GO UP BIG